Method of closing off a heat pipe

ABSTRACT

A method of closing off a heat pipe of hydrogen-permeable material subsequent to evacuation of undesirable gases, the heat pipe is filled with hydrogen, is then sealed, and finally the hydrogen is removed by diffusion through the wall of the pipe.

United States Patent 1191 Asselman 1 Mar. 19, 1974 METHOD OF CLOSING OFF A HEAT PIPE [56] References Cited [75] Inventor: George Albert Apolonia Asselman, UNITED STATES PATENTS Emmasing l, E h n, 3.033.974 5/1962 Schleich et a1 219/117 R Netherlands 3.503.438 3/1970 Geyer 165/105 3.604.504 9/1971 Kessler et a1. 165/105 X [731 Asslgnee= Cmpmamn, New 3.613773 10/1971 H311 et a1. 165/105 x York, N.Y. I [22] Filed: Nov. 12, 1972 Primary ExaminerCharles W. Lanham Assistant ExaminerD. C. Reiley, Ill [21] Appl' 308765 Attorney, Agent, or FirmFrank R. Trifari [30] Foreign Application Priority Data 57 1 ABSTRACT Decv 3. 1971 Netherlands 1. 7116620 A method of closing Off a heat p p of y g permeable material subsequent to evacuation of unde- "29/1573 29/DIG 53/]2 sirable gases, the heat pipe is filled with hydrogen, is 1m CT 1. uhmmniiasirii then sealed, and finally the hydrogen is removed by diffusion throu h the wall of the i e. [58] Field of Search. 165/105; 29/1573 R, DIG. 34; g p p 6 Claims, 3 Drawing Figures METHOD OF CLOSING OFF A HEAT PIPE The invention relates to a method of closing off a heat pipe, in which a sealing place is sealed while heating it to at least its softening temperature.

A heat pipe within the scope of the present application is to be understood to mean a heat transfer device constituted by a container in which a heat transfer medium is present, for example sodium, which on the one hand absorbs thermal energy from a heat source through a wall of the container while changing from the liquid phase into the vapour phase and on the other hand delivers thermal energy to the exterior through another wall of the container while changing from the vapor phase into the liquid phase. Such a heat pipe permits of transporting large quantities of thermal energy substantially without temperature drop and without a pumping device or other moving parts.

Return of condensed heat transfer medium to the wall where the evaporation takes place may occur under the influence of gravity. Often, however, a capillary structure is present in the heat pipe and communicates the condensation wall with the evaporation wall and through which condensate is returned to the evaporation wall in all circumstances on the basis of capillary action. So return takes place also against gravity or without the gravity field being present.

Heat pipes comprising a capillary structure for the return of condensate are known, for example, from the US. Pat. Nos. 3,229,759 and 3,402,767. For a good operation of the heat pipe it is desirable to remove all foreign gases, including air, from the heat pipe. As a matter of fact, such gases can cause all kinds of difficulties. They can inhibit, for example, the condensation of heat transfer medium on the condensation wall in that said wall is covered with a gas layer or they can enter into chemical reactions with the heat transfer medium, the material of the capillary structure or that of the heat pipe walls.

Undesirable gases which would be liberated from the heat pipe walls or the capillary structure during the often high operating temperature of the heat pipe, can be eliminated for the greater part in advance by subjecting the heat pipe to a thermal treatment, for example, annealing in a vacuum furnace, before filling it with heat transfer medium and sealing it.

Closing off the heat pipe can be done by means of cocks. On the one hand, this makes the heat pipe comparatively expensive, on the other hand a hermetic seal is often not obtained since the cock is apt to show leakage. Undesirable gases then leak into the heat pipe and may cause again the above-mentioned difficulties.

Since closing off the heat pipe is in most of the cases a one-time job, a sealing method is preferably used so as to obtain a good sealing, for example melting, soldering or welding, the place to be sealed obtaining the shape desirable for sealing by heating to at least its softening temperature (contraction of a filling spout and exhaust tube, respectively).

It is quite a problem to obtain in a simple manner a hermetic seal of a heat pipe which comprises a heat transfer medium and is otherwise evacuated, so liberated from undesirable gases.

Upon sealing in an atmosphere of atmospheric pressure, implosion of the heat pipe easily occurs in the place to be sealed when said place is softened while a vacuum prevails in the heat pipe. The danger also exists that gases, such as air, flow into the heat pipe via the place to be sealed and spoil the vacuum. In addition, the evacuated heat pipe often assumes such a high temperature as a result of the heating of the place to be sealed that the heat pipe is difficult to handle.

It is possible to mitigate the above-mentioned drawbacks by producing the seal by means of electron beam welding and soldering, respectively, in a vacuum. However, such a method is time-consuming and expensive and moreover requires expensive apparatus. When using an electron beam welding apparatus as is known, for example, from the US. Pat. No. 3,033,974, each time only one heat pipe can be welded in the apparatus. The heat pipe should be arranged accurately in the treatment chamber. Electron beam welding can take place after evacuation of the treatment chamber. After removing the vacuum in the treatment chamber, the

heat pipe may be removed from it. In view of the timeconsuming procedure in welding with the help of beams of charged particles and the required expensive apparatus, said method is unattractive for economic considerations.

It is the object of the present invention to provide a .simple and cheap method in which hermetically sealed and, with the exception of the heat transfer medium filling, evacuated heat pipes are obtained without the danger of implosion and in a handy manner.

The method according to the invention is characterized in that after evacuation of undesirable gases, the heat pipe is filled with hydrogen, is then sealed, after which the hydrogen is removed to the atmosphere by diffusion through the wall of the heat pipe.

Due to the hydrogen filling there is no danger of implosion of the heat pipe upon scaling in an atmosphere in which the ambient pressure makesitself felt in the place to be sealed. Sealing can thus be carried out simply and easily in the normal atmospheric atmosphere.

Moreover the hydrogen during sealing ensures that the temperature of the heat pipe does not become too high because evaporation of the heat transfer medium and flow of medium vapour to the colder parts of the pipe is prevented. The heat pipe then remains easy to handle. Sealing may thus be carried out as handwork.

Finally, an evacuated heatpipe is ultimately obtained again in a simple manner by causing the hydrogen to diffuse from the heat pipe to the atmosphere. It has actually been found that all conventional heat pipe materials permit of diffusion of hydrogen through the pipe walls. Materials used for the manufacture of heat pipes are, for example, glass, ceramic, steel, the metals copper, aluminum, molybdenum, niobium, zirconium, tungsten, tantalum, rhenium and alloys thereof. The hydrogen may be introduced into the heat pipe in the form of a gas. If desirable, however, liquid hydrogen may also be supplied which evaporates in the heat pipe and provides an extra cooling of the heat pipe due to its low temperature.

In a favourable embodiment of the method according to the invention the hydrogen pressure in the heat pipe during sealing is equal to or substantially equal to the ambient pressure. The ambient pressure is usually the atmospheric pressure but this need not always be the case. Since now the pressures on either side of the place to be sealed are equal or substantially equal, neither the danger of implosion nor of explosion will exist and a good sealing will be obtained.

The hydrogen inside the heat pipe disappears from it by diffusion through the walls of the heat pipe to the atmosphere. Dependent upon the material of the heat pipe and the temperature thereof, said hydrogen diffusion will occur more or less rapidly. In order to increase the diffusion rate of the hydrogen, according to the invention the heat pipe is heated. By heating, the wall temperature of the heat pipe rises and the hydrogen pressure inside the heat pipe increases. Both factors have a favourable influence on the hydrogen diffusion rate. When the nominal operating temperature of the heat pipe is so high that such a temperature can be a positive contribution to increasing the hydrogen diffusion rate, the heat pipe may also be made free from hydrogen by operating it in the normal practical circumstances endeavoured.

As already said, the diffusion rate of hydrogen depends upon the material of the heat pipe. In connection with machinability, operating temperature, affinity to heat transfer medium and so on of the heat pipe material or from the point of view of manufacture and design it may be desirable or necessary that the starting material chosen is a material which has a comparatively low permeability for hydrogen.

According to the invention, an increase of the hydrogen diffusion can nevertheless be obtained by using a heat pipe of which a part of the material of comparatively low permeability of hydrogen is replaced, preferably in the form of a window, by a material having a larger permeability for hydrogen.

Materials through which hydrogen can diffuse at a high rate, notably at higher temperatures (above 600C), are palladium or alloys thereof, such as silverpalladium and (to a slightly smaller extent) nickel or alloys thereof, for example, nickel-iron alloys. By using said materials as windows in heat pipes which consist of a material having a comparatively low permeability to hydrogen, the great advantage is obtained that upon heating the hydrogen is expelled from the heat pipe in a very short time via the window. The invention furthermore relates to a heat pipe manufactured according to the above-described methods.

The invention will be described in greater detail, by way of example, with reference to the drawings which are diagrammatic and not drawn to scale.

BRIEF DESCRIPTION OF THE DRAWINGS FIG. la is an elevation view partially in section of the new heat pipe assembly.

FIG. lb is a sectional view of a portion of FIG. 1a in further detail.

FIG. 1c is a sectional view of a portion of FIG. la in further detail.

DESCRIPTION OF THE PREFERRED EMBODIMENT Reference numeral 1 in FIG. la denotes a stainless steel heat pipe which is provided with a nickel window 2 and a filling spout 3. The inner wall of the pipe is lined with a capillary structure 4 consisting of layers of gauze. A duct 5 in which a three-way cock 6 is incorporated, communicates with the filling spout 3. By means of the three-way cock 6, the heat pipe 1 may be made to communiate at will with a vacuum pump 7 or a hydrogen cylinder 8 filled with hydrogen under pressure and comprising a pressure reducing valve 9. The heat pipe 1 comprises a quantity of sodium as a heat transfer medium which may have been provided in the heat pipe, for example, via a distillation process.

Prior to the sodium filling, the heat pipe 1 has been heated with simultaneous evacuation by a vacuum pump 7 so as to remove all the undesirable gases from the heat pipe. If desirable, heating may be repeated after the sodium filling so that undesirable gases which are liberated from the sodium are then also removed.

The heat pipe is then rinsed once or several times with hydrogen from the hydrogen cylinder 8, the hydrogen being sucked off after each rinsing by vacuum pump 7. The heat pipe 1 is then filled with hydrogen of approximately l atm. and the filling spout 3 is sealed,

in this case by contraction and melting Substantially the same pressure prevails on either side of the filling spout 3 so that melting occurs without trouble and without the danger of implosion and a good seal is obtained.

The hydrogen in the heat pipe 1 ensures that during sealing the temperature of the heat pipe remains low. As a matter of fact, the hydrogen restricts evaporation of heat transfer medium so that medium vapor cannot condense on colder parts of the wall. As a result of this the heat pipe remains readily handable. After sealing the heat pipe is heated. In the present case this is done with an electric heating coil 10 as is shown in FIG. 1b. The nickel window 2 and the walls of the heat pipe 1 are heated at a high temperature. The hydrogen pressure inside the heat pipe increases. Hydrogen now diffuses at a high speed through the nickel window 2 to the exterior. Diffusion of hydrogen also takes place through the stainless steel walls of the heat pipe, albeit at a much lower rate than is the case for the nickel window. Within a few minutes the heat pipe is free from hydrogen and ready for normal use.

In normal use of the heat pipe, thermal energy is supplied to the part A of the pipe (FIG. 1c). The sodium inside the heat pipe evaporates there and flows to the part B of the pipe in the vapor phase because this part has a slightly lower temperature so that a lower vapor pressure prevails at the area of pipe part B. Sodium vapor condenses on pipe part B while giving off thermal energy to said part. Through the capillary structure 4 formed by layers of gauze, the condensate is returned to pipe part A, on the basis of capillary action, to be evaporated there again.

Should any hydrogen have remained in the heat pipe it is driven in the direction of the nickel window 2 by the sodium vapor. Via diffusion through the window 2, said remaining hydrogen then disappears to the atmosphere. Although in this example the window 2 is provided in an end face of the heat pipe, all kinds of other places are possible, of course.

What is claimed is:

1. A method of closing off a heat pipe, at least a part of which is hydrogen permeable and in which a sealing place is sealed while heating it to at least its softening temperature, said method comprising evacuating undesirable gases from the heat pipe, filling the evacuated heat pipe wth hydrogen, then sealing the heat pipe, and subsequently removing the hydrogen into the atmosphere by diffusion of the hydrogen through the wall of the heat pipe.

the heat pipe.

5. A method as claimed in claim 4, wherein the window consists of nickel or an alloy thereof, for example, an iron-nickel alloy.

6. A method as claimed in claim 4, wherein the window consists of palladium or an alloy thereof, for example, a silver-palladium alloy. 

1. A method of closing off a heat pipe, at least a part of which is hydrogen permeable and in which a sealing place is sealed while heating it to at least its softening temperature, said method comprising evacuating undesirable gases from the heat pipe, filling the evacuated heat pipe wth hydrogen, then sealing the heat pipe, and subsequently removing the hydrogen into the atmosphere by diffusion of the hydrogen through the wall of the heat pipe.
 2. A method as claimed in claim 1, wherein the hydrogen pressure in the heat pipe during sealing is equal to or substantially equal to the ambient pressure.
 3. A method as claimed in claim 1 wherein the heat pipe is heated in order to increase the rate of the hydrogen diffusion to the atmosphere.
 4. A method as claimed in claim 1, wherein the heat pipe is provided with a window of a material which has a larger permeability for hydrogen than the material of the heat pipe.
 5. A method as claimed in claim 4, wherein the window consists of nickel or an alloy thereof, for example, an iron-nickel alloy.
 6. A method as claimed in claim 4, wherein the window consists of palladium or an alloy thereof, for example, a silver-palladium alloy. 